1. Field of the Invention
The present invention pertains to recording on magnetic tape, and particularly to correcting for distortion in write data timing caused by using a direct current (DC) content code with a transformer-coupled system.
2. Related Art and Other Considerations
In magnetic recording on tape using a magnetic tape drive, relative motion between a head unit (typically with both a write element and a read element) and the tape causes a plurality of tracks of information to be transduced with respect to the tape. The magnetic tape is typically housed in a cartridge which is loaded into the tape drive. The tape extends between a cartridge supply reel and a cartridge take-up reel. The tape drive typically has a supply reel motor for rotating the cartridge supply reel and a take-up reel motor for rotating the cartridge take-up reel.
After the cartridge is loaded into the tape drive, the tape is extracted by mechanisms in the drive so that a segment of the tape is pulled from the cartridge and into a tape path that travels proximate the head unit. The extraction mechanisms take the form of tape guides which are mounted on trolleys. During the extraction operation, trolley motors move the trolleys along a predefined trolley path, so that the tape guides which surmount the trolleys displace the tape into the tape path as the trolleys travel along the trolley path. When the trolleys reach the full extent of travel along the trolley path, the tape is proximate the head unit. Thereafter the tape can be transported past the head unit, e.g., by activation of a capstan and/or the supply reel and take-up reel motors, depending upon the particular type of transport mechanisms employed.
In a helical scan arrangement, as the magnetic tape is transported the magnetic tape is at least partially wrapped around a rotating drum so that heads (both write heads and read heads) positioned on the drum are contiguous to the drum as the drum is rotated. One or more write heads on the drum physically record data on the tape in a series of discrete stripes oriented at an angle with respect to the direction of tape travel. The data is formatted, prior to recording on the tape, to provide sufficient referencing information to enable later recovery during readout by one or more read heads. Examples of helical scan tape drives are shown, inter alia, in the following U.S. patents all of which are incorporated herein by reference): U.S. Pat. No. 4,835,628 to Hinz et al.; U.S. Pat. No. 4,843,495 to Georgis et al.; U.S. Pat. No. 5,065,261 to Hughes et al.; U.S. Pat. No. 5,068,757 to Hughes et al.; U.S. Pat. No. 5,142,422 to Zook et al.; and U.S. Pat. No. 5,602,694 to Miles et al. (which discloses a capstanless helical scan tape drive).
As the tape is transported past the head unit, information can be transduced to or from the tape by the tape drive in recording and reading operations, respectively. In many tape drives, the data to be written or recorded on tape is applied to a write head of the head unit through a transformer having a high pass element.
When the recording and/or reading operations are concluded, and before the cartridge can be unloaded from the drive, the tape must be retracted for return to the interior of the cartridge. Tape retraction is essentially the reverse of the tape extraction procedure described above.
In recording information on magnetic tape, various mathematical codes can be employed for error detection and/or error correcting purposes. The recording process is inductive in nature, and if the code used to record the information does not supply a run length constraint then the data may become unreadable. This is because no clock signal is recorded; rather the clock is reconstructed out of the recorded data. This implies that if no transitions are present then there is nothing to use to reproduce the clock, hence the need for a run length constraint.
In a helical scan recorder some means of coupling to the heads on the rotating drum is required. This coupling often takes the form of a rotary transformer. Typically the rotary transformer creates a high pass response in the write data path. Traditionally the problem of high pass response has been corrected by use of a DC free code, as DC information cannot be passed through the transformer. The use of a DC free code is not without it""s problems, however. In order to achieve a DC free code, constraints are applied to the code that limit it""s density, increasing the overhead associated with using the code. This implies that if a method is discovered to write a DC content code on an AC coupled write path then a higher effective density can be achieved (xcx9c15%).
FIG. 12A and FIG. 12B illustrate problems attending the use of a DC code in a transformer-coupled tape drive system. FIG. 12A shows a worst case write waveform before the transformer; FIG. 12B shows the write waveform after the transformer. The waveforms shown in FIG. 12A and FIG. 12B are for both passive and active systems, the difference is that the waveforms represent a current waveform in the passive case, and both waveforms represent a voltage waveform for the active case. For the passive case, FIG. 12A shows the current in the transformer primary and FIG. 12B shows the current in the secondary and head. For the active case, FIG. 12A shows the voltage on the transformer primary and FIG. 12B shows the voltage on the transformer secondary, as well as being the voltage input to the write driver. As used herein, a xe2x80x9cpassivexe2x80x9d system has only a write head on the rotating side of the transformer, and that the waveform is the write current through the head. An active system, on the other hand, includes a write current driver to drive the write head, and the waveform is the voltage present to the input of the write current driver. It is incidental that power on the rotor (POR) is included, the POR enables incorporation of write drivers on the rotating side of the transformer.
As shown in FIG. 12A and FIG. 12B, the result of the transformer in this case is to modify the times such that T1 has increased, and T2 has decreased. The dotted line across each of the two waveforms is the zero (switching threshold) point. Note that the action of the high pass of the transformer is to make of the area on each side of the zero line equal. This characteristic is the problem with writing a DC content code: if the system is passive then the write current in each direction becomes a function of the code""s DC content. In this example much more current is being used to write in the upward direction than the down going direction. This difference is high enough that the head may be saturated in the up going direction, and may not be able to overwrite the old data in the downward one.
A slow write waveform rise time is employed in FIG. 12A and FIG. 12B in order to better illustrate a further problem. In FIG. 12A and FIG. 12B the zero crossings have moved out from the center of the narrow pulses, resulting in a xe2x80x9cpulse pairingxe2x80x9d. This movement is on the order of 0.7 nsec worst case using a 16/17 code. This is almost 10% of the window, which is enough error to prevent the tape drive from being able to read data recorded with the DC code.
It might be proposed to drive the transformer in such a fashion that the DC offset never builds up on it""s output. Integrating the DC offset into the transformer using the code""s DC content conceivably could accomplish this. However, as the maximum DC content of the code is not constrained there is also no constraint on the drive level. Therefore, this proposal is not realizable.
What is needed, therefore, and an object of the invention, is a transformer-coupled tape drive system which uses a DC code.
A transformer-coupled tape drive operates using a direct current (DC) content code. Usage of the DC content code is facilitated by a compensation circuit which corrects timing distortion caused by the DC content code. An advantage of allowing DC content is that the maximum amount of data that can be recorded is increased for a given head/tape combination.
In the tape drive of the present invention, an encoding unit encodes data to be recorded with a direct current (DC) content code, and generates a write data signal for DC-code encoded data. The write data signal is applied to a compensation circuit of the present invention, which outputs a compensated write data signal (which is corrected for the distortion caused by the DC content code). The compensated write data signal is then applied to a high pass transformer, which outputs a transformed write data signal. A write head on a rotating scanner or drum records information on the magnetic tape in accordance with the transformed write data signal.
The compensation circuit comprises both an emulation circuit and a delay circuit. The emulation circuit emulates the high pass filter action of the transformer to provide an emulated high pass filter response signal. The delay circuit modifies the timing of the write data signal in accordance with the emulated high pass filter response signal to output the compensated write data signal. That is, the delay circuit modifies the positions of the zero crossings of the write data signal in such a fashion that the zero crossings are in the correct position after the transformer.